The invention relates to systems for sensing deviation of a surface, structure or the like from a reference plane which may be, for example, a horizontal plane.
In such systems it has been known to use a tilt sensitive transducer which has two resistance values which vary according to the angle and direction of tilt. When the transducer is connected in an A.C. bridge circuit an output signal varying in amplitude and phase according to the angle and direction of tilt can be obtained. See, for example, U.S. Pat. No. 3,114,209 which issued Dec. 17, 1963 to Foody et al. In that patent there is no teaching of how the phase of the output signal is actually determined so as to determine the direction of tilt. Moreover, it is noted that the Foody circuit disclosed does not provide a reference voltage in terms of magnitude and phase and this presents a problem in processing the output signal.